1. Field of the Invention
The present invention relates to a piezoelectric transformer type high-voltage power supply device and an image forming apparatus and, more particularly, to a technique of controlling an output voltage or an output current.
2. Description of the Related Art
A conventionally known electrophotographic image forming apparatus uses a DC bias voltage as a voltage to be applied to the transfer member. To generate a high voltage necessary for image formation, a wire-wound electromagnetic transformer has been used conventionally. However, the output current value of the electromagnetic transformer used in the image forming apparatus of the above-described specifications is as small as several μA. Hence, the leakage current needs to be minimized in every unit. To do this, the winding of the transformer needs to be insulated by molding or the like. In addition, the transformer needs to be relatively large to its supply power. For these reasons, it is difficult to reduce the size and weight of the high-voltage power supply device.
To solve these problems, a proposal has been made to generate a high voltage using a slim and lightweight high-power piezoelectric transformer (Japanese Patent Laid-Open No. 11-206113). More specifically, using a piezoelectric transformer made of a ceramic makes it possible to generate a high voltage at an efficiency higher than that of the electromagnetic transformer and also increase the distance between the primary side electrode and the secondary side electrode. Since the special molding for insulation is unnecessary, the high-voltage generator can be made compact and lightweight.
Japanese Patent Laid-Open No. 11-206113 discloses a high-voltage power supply device which causes a voltage controlled oscillator (VCO) formed from an analog circuit to generate a driving frequency to be input to the piezoelectric transformer. As a feature of the piezoelectric transformer, its output voltage is maximized at the resonance frequency. It is therefore possible to control the output voltage by the frequency. Note that as the features of the relationship between the driving frequency and the output voltage, the output voltage is maximized at the resonance frequency and lowers as the frequency becomes higher or lower than the resonance frequency. The high-voltage power supply device described in Japanese Patent Laid-Open No. 11-206113 controls the frequency output from the VCO, thereby controlling the output voltage of the piezoelectric transformer.
Recently demanded is space saving using fewer components. As described above, the control circuit portion of the piezoelectric transformer is formed from an analog circuit in most cases and therefore includes many components. To decrease the number of components of the control circuit portion and implement a space-saving piezoelectric transformer, the control circuit portion is formed as an IC on one chip. However, since the number of pins of an IC is limited, the IC may be unusable in a small package. In addition, when the package is small, and ten-odd piezoelectric transformers are used as in a color printer, a plurality of control ICs are necessary, and a large space-saving effect is difficult to obtain. In this case, the ICs are formed on one chip together with the CPU and the ASIC of the controller of the printer engine unit. In this method, a large space-saving effect can be obtained. The control circuit can be formed from not an analog circuit as before but a digital circuit. However, when the control circuit portion of the piezoelectric transformer is digitized, driving control of the piezoelectric transformer and, more particularly, output voltage control by a variable frequency requires a high frequency accuracy. That is, for accurate frequency control, the CPU or the ASIC needs to have a very high operation clock speed.
For example, when a 10-bit counter (the MAX count is 1024) counts 602 pulses in both H and L levels (the count is 1204 in one period) for pulse generation, the output frequency value is 166.113 KHz. When 603 H pulses and 602 L pulses are counted (the count is 1205 in one period), the output frequency value is 165.975 KHz. The frequency difference Δf is 120 Hz. When the frequency changes by 100 Hz, the output voltage changes by about 20 V. To avoid any influence on an image, the voltage needs to change at an accuracy of 2.0 V or less. To change the voltage at an accuracy of 1.5 V or less, a frequency resolution of 10 Hz or less is necessary. In that case, the frequency of the operation clock of the CPU or the ASIC needs to be 2 GHz or more. That is, examples of problems posed by speeding up the internal operation clock are an increase in unwanted radiation noise, higher power consumption, and an increase in the cost caused by the semiconductor microfabrication process.